Present day IMDs such as cardiac pacemakers or ICDs incorporate electrical stimulation pulse generators as well as cardiac event sensors that can pace, sense, and/or shock the tissue of the atrium, ventricle, or both the atrium and ventricle of the heart.
Noninvasive telemetry has been developed allowing information such as data and control signals to be bidirectionally communicated, for example, by means of a radio frequency (RF) coupling, between an IMD and an external system. Such an external system, typically comprising a controller, a programmer, and/or a monitor, provides a convenient means through which the operation of the IMD can be controlled and monitored, and through which information sensed by the IMD can be read, interpreted, or otherwise used.
In an RF-coupled system information is transferred from a transmitting antenna to a receiving antenna by way of a radiated carrier signal. The carrier signal is modulated with the data to be transmitted using an appropriate modulation scheme. The modulated carrier induces a voltage in the receiving antenna that tracks the modulated carrier signal. The received signal is then demodulated to recover the transmitted data.
Present day IMDs are enclosed within hermetically sealed casings, typically made of a titanium alloy selected for its high strength, corrosion-resistance, biocompatibility and biostability. An IMD's RF telemetry circuitry comprising, for example, a ferrite core, a wire coil and an RF antenna, is not biocompatible, and therefore must be placed inside the hermetically sealed casing. RF-coupled telemetry through an IMD casing is affected by the properties of the casing, including its material, thickness and geometry. In particular, the rate at which telemetry is possible is largely determined by the electrical conductivity and the thickness of the casing. There has been a significant increase in the complexity of IMDs so that the quantity of information that now must be telemetrically transferred between an IMD and its associated external system has dramatically increased.
Electrical eddy currents great enough to adversely attenuate the radiated RF field and limit the information transfer distance and rate between the IMD and the associated external system can be generated in a metal casing having a high electrical conductivity. A titanium casing, in particular, acts as a low pass filter, attenuating high frequencies so that the carrier frequency cannot be increased above approximately 10–20 kHz without an unacceptable increase in transmitter power. Biocompatible materials having significantly lower electrical conductivities are not available without manufacturability and corrosion problems.
Thus, there continues to be a need for more efficient ways to achieve high-speed telemetric information transfer to and from an IMD. At the same time, it would be advantageous to continue to use a casing material such as titanium that offers the required biostability, biocompatibility, corrosion-resistance, manufacturability and structural strength.